In order to control large machines or plants, it is necessary to connect an electronic control or Central Processing Unit (CPU) with a number of reporting, positioning and measuring peripherals. This CPU is usually not in the immediate vicinity of the peripherals. In order to obviate the need to run separate lines from each of these connected devices to the CPU, it is known to provide, in the immediate vicinity of the machine or controlled device, one or more electronic input/output systems, which are connected to the CPU by a data bus for transmission of signals in both directions. Such systems are disclosed in U.S. Pat. Nos. 4,870,564 and 4,764,868, both Ketelhut et al, to which German Patent Disclosure Document DE-OS 35 19 807 corresponds.
In general, electronic controls are located outside the range of influence of dust, cuttings, and/or moisture generated at the machine or installation. An input/output system is, however, often exposed to such influences, and/or must be located in a hard-to-access location.
Therefore, an input/output system must be enclosed in a housing which is made resistant to dust and/or water. Into this housing are fed the large number of connectors for the reporting, positioning, and measuring units, and the power supply lines for the input/output system. Generally, all these equipment elements, arranged on or in the machine or installation, are subjected to harsh environmental conditions, so the most frequent errors or faults occur at these elements. In order to recognize and localize these faults in a timely manner, it is necessary to provide circuits which enable monitoring and supervision of all these connections and/or the operation of the reporting, positioning, and measuring units, e.g. as disclosed in U.S. Pat. No. 4,593,380, GAREIS et al./GENERAL ELECTRIC, corresponding to German DE-OS 35 19 826, and DE-OS 34 40 025 (CHWIERALSKY et al./PULSOTRONIC). In case of trouble, generally the source of the fault, and, in part, the type of fault, e.g. loose connection, short-circuit, or faulty operation, are displayed.
In spite of these aids, the actual error correction is difficult in many cases, since the fault location may not be unambiguously identified, and the replacement of individual reporting, positioning, or measuring units or their connections to the input/output system is not only laborious, but can only be carried out by specially trained experts. This is the case, above all, when a fault determination or fault correction in the input/output system must be carried out, or when it is necessary to replace a reporting, positioning, or measuring unit and its connecting lines to the system.